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Abstract

Label-free, single-object sensing with a microring resonator is investigated numerically using the finite difference time-domain (FDTD) method. A pulse with ultra-wide bandwidth that spans over several resonant modes of the ring and of the sensing object is used for simulation, enabling a single-shot simulation of the microring sensing. The FDTD simulation not only can describe the circulation of the light in a whispering-gallery-mode (WGM) microring and multiple interactions between the light and the sensing object, but also other important factors of the sensing system, such as scattering and radiation losses. The FDTD results show that the simulation can yield a resonant shift of the WGM cavity modes. Furthermore, it can also extract eigenmodes of the sensing object, and therefore information from deep inside the object. The simulation method is not only suitable for a single object (single molecule, nano-, micro-scale particle) but can be extended to the problem of multiple objects as well.

Figures (8)

Schematic of a microring sensor: a ring resonator (radius R, width dR and index nR) coupled to a waveguide (width dW, index nW). The gap between the ring and the waveguide is g. A sensing object (radius rSO and index nSO) is adjacent to the ring (light blue microdisk).

FDTD simulation of light field with frequency of 192 THz in a microring with R = 20μm, nR = nW = 1.46, dR = dW = 1μm . (a): without sensing object, and (b) with sensing object rSO = 2 μm, nSO = 3.6. The long waveguide around the sensing system acts to minimize the boundary reflection that can couple back into to the cavity during the resonant recirculation of the light.

A general result from FDTD simulation of the pulse propagating in the waveguide and coupling to the microring. (a) E-field of the input pulse in time domain (TD), and (b) normalized intensity in frequency domain (FD). (c) E-field inside the cavity measured at position 2 in TD, and (d) relative intensity in FD showing the resonant modes of the ring without the sensing object. (e) E-field in TD and (f) transmission in FD (at position 3). Central: intensity of the light with frequency f = 192 THz which is closest to a resonant mode of the ring shown in (d).

Eigenmodes of several SOs with different size and indices within the bandwidth of the light pulse that was used to simulate the microring sensing above. (a) r = 1μm, n = 3.6, (b) r = 2μm, n = 3.6, (c) r = 1μm, n = 4, and (d) r = 2μm, n = 2.0. The two SOs in (a) and (b) are used for the simulation of microring sensing shown in Figs. 4(b) and 4(c) above. Media 3 (or in large format, Media 4) is a short animation of light propagation in SO with r=2 m, n=3.6 (e.g., Fig. 5(b)).

(a): Transmission T of the micro-sensing without (red) and without sensing object (blue). The microring (R = 20 μm, dR = 1μm, nR = 1.46); SO rSO = 2 μm and nSO = 3.6., and (b): Relative intensity inside microring without (red) and with (blue) sensing object, and inside the sensing object (green) which show the resonant modes in the systems. The SO has rSO = 2 μm and nSO = 3.6. The two green arrows indicate new modes that come from SO and can be detected in the transmission.

During the recirculation of light in the ring, each time it passes the coupling region, light can couple to and subsequently co-propagates in the waveguide. Left: first time, (central) second time, and (right) third time light passes the coupling region. (See the propagation of light in the system for the whole time in the Media 1).